I like the mining claims, but I think we need to make sure we understand what we're being given. If you just want dilithium, you'd do better taking your 125 Zen and trading it for Dil on the exchange. I consider them, however, a sort of consolation prize that takes some of the sting out of buying keys. You open a bunch of boxes, maybe you get something good and maybe you don't, but odds are you got some mining claims which can help you earn some of your money back.
Again, I think they're a good addition to the boxes, but don't open Dominion lockboxes just to get them. That would be counterproductive.
While Cryptic refused to publish tables and odds of items in the lock boxes, I have no such hesitancy.
With the assistance of another forum member, I opened 1,000 Dominion lockboxes when they were first available. You can see the results posted (as a table) in my signature.
The odds my have been tweaked slightly since the box was launched. There were also Lobi Crystals added to the box. You can still get a good sense of what is inside these boxes from looking at my results.